The current research investigates the effect of birth order on the social and exploratory behavior of young children. The children are firstborns about 18 months of age and like-aged secondborns whose siblings are two to four years older. In a free play situation in the laboratory we compare their responses to their mothers, unfamiliar adults, siblings or unfamiliar children matching the siblings in age and sex. Attention is directed not only to the behavior of the 18-month-old children but also to the responses of the other participants to them, and to the semantic properties of their verbal discourse. We expect to find less dependent behavior toward the mothers and more positive responses to unfamiliar adults in firstborns. Secondborn, children, similarly, are predicted to interact more frequently with unfamiliar children of their siblings' age and sex than are firstborns. BIBLIOGRAPHIC REFERENCES: Rheingold, H.L., Hay, D.F., & West, M.J. Sharing in the second year of life. Child Development, 1976, 47, 1148-1158. Rheingold, H.L. Discussant's comments. In T.D. Tjossem (Ed.), Intervention strategies for high risk infants and young children. Baltimore: University Park Press, 1976. Pp. 141-142.